In many applications and primarily in audio reproduction systems, for example in car audios, HI-FI audio systems and similar apparatuses that are intrinsically compact because of stringent installation requirements, as well as in portable apparatuses, power dissipation in final power stages, often quadrupled in order to drive a pair of loudspeakers (front and rear) for each stereo channel, may create heat balance problems. For example, four 20 W amplifiers may have a power dissipation of about 4×12=48 W and because of the limited space available in certain apparatuses, such a relatively large power may be difficult to dissipate without a significant increase of temperature within the apparatus.
On the other hand, a relatively high temperature of operation may degrade the magnetic tape of cassettes or optical disks (CD), the drives of which are often tightly fitted inside a single apparatus case.
The so-called D-type switching amplifiers are highly efficient and are considered the most appropriate type for these applications.
Unfortunately, switching amplifiers generate electromagnetic emissions that in compact apparatuses may interfere with the correct functioning of other systems, reducing their performances. For these reasons, audio signals are frequently amplified using a pair of class AB power amplifiers, operating in single-ended or in bridge configuration depending on the level of the processed signal.
In fact, class AB power amplifiers are less efficient than switching amplifiers and a common technique for reducing power consumption of class AB amplifiers consists in configuring them in single-ended instead of in bridge configuration, whenever it is possible to do so. In fact, these amplifiers dissipate more power in bridge configuration than in single-ended configuration as long as the amplitude of the output signal remains smaller than the positive supply voltage. Unfortunately, it is not possible to use single-ended class AB amplifiers if the output surpasses this voltage because the output signal would be severely distorted by clipping.
Techniques for automatically switching from one configuration to the other in function of the monitored level of the signal are implemented in the commonly owned U.S. Pat. No. 5,194,821, U.S. Pat. No. 5,365,188 and U.S. Pat. No. 5,654,688.
The U.S. Pat. No. 5,194,821 discloses a bridge amplifier using a positive and a negative supply voltage sources, that may function in single-ended or in differential or bridge output configuration, depending on the level of the output signal. Substantially, a comparator changes the output circuital configuration of the amplifier from a bridge configuration to a single-ended configuration or vice versa by closing or opening configuring switches, when the output signal becomes smaller than or greater than a certain threshold voltage.
The U.S. Pat. No. 5,365,188 and U.S. Pat. No. 5,654,688 disclose a single supply dual bridge power amplifier, having a window comparator for sensing the level of input signals fed to the amplifier and driving the switches that coordinately configure the amplifier in either a bridge or in a single-ended configuration.
A system of the type disclosed in the above mentioned patents is schematically shown in FIG. 1.
Four operational amplifiers OP1+, OP1−, OP2+, OP2− are respectively input with the signals Ch1 and Ch2 for driving two loudspeakers. A window comparator is input with the two signals Ch1 and Ch2 and positions the switches that connect the loudspeaker of the channel Ch2 either to the output of the operational amplifier (OP2+) or to a certain reference voltage VREF. The operational amplifier OP1− is configured by the window comparator that positions the path-selector shown within the dotted perimeter for functioning as a voltage buffer outputting the reference voltage VREF, by coupling an input thereof to a fixed voltage VF.
In the scheme of FIG. 1, the switches of the power amplifier are shown in the position that configures the amplifier with two single-ended channels driving the respective loudspeakers.
In car audio systems two or more pairs of amplifiers of this kind are often used for driving four loudspeakers FR (Front Right), FL (Front Left), RR (Rear Right) and RL (Rear Left) through independently equalizable channels.
Surprisingly, it has been noticed that frequently the efficiency of this multi-channel power amplifier inexplicably drops and crosstalk effects increase.